Advanced steering tool system, method and apparatus

ABSTRACT

A steering tool is movable by a drill string to form an underground bore along an intended path. A sensing arrangement of the steering tool detects its pitch and yaw orientations at a series of spaced apart positions along the bore, each position is characterized by a measured extension of the drill string. The steering tool further includes a receiver. At least one marker is positioned proximate to the intended path, for transmitting a rotating dipole field to expose a portion of the intended path to the field for reception by the receiver. The detected pitch orientation, the detected yaw orientation and the measured extension of the drill string are used in conjunction with magnetic information from the receiver to locate the steering tool. The steering tool may automatically use the magnetic information when it is available. A customized overall position determination accuracy can be provided along the intended path.

RELATED APPLICATIONS

This application is a continuation application of copending U.S. patentapplication Ser. No. 16/396,748 filed on Apr. 28, 2019, which is acontinuation application of U.S. patent application Ser. No. 15/720,865filed on Sep. 29, 2017 and issued as U.S. Pat. No. 10,273,796 on Apr.30, 2019, which is a continuation of U.S. application Ser. No.15/158,456 filed on May 18, 2016 and issued as U.S. Pat. No. 9,777,567on Oct. 3, 2017, which is a continuation application of U.S. patentapplication Ser. No. 15/004,783 filed on Jan. 22, 2016 and issued asU.S. Pat. No. 9,360,579 on Jun. 7, 2016, which is a continuationapplication of U.S. patent application Ser. No. 14/821,599 filed on Aug.7, 2015 and issued as U.S. Pat. No. 9,244,189 on Jan. 26, 2016, which isa continuation application of U.S. patent application Ser. No.14/163,374 filed on Jan. 24, 2014 and issued as U.S. Pat. No. 9,133,703on Sep. 15, 2015, which is a divisional application of U.S. patentapplication Ser. No. 12/816,250 filed on Jun. 15, 2010 and issued asU.S. Pat. No. 8,659,298 on Feb. 25, 2014, which is a continuationapplication of U.S. patent application Ser. No. 11/835,154 filed on Aug.7, 2007 and issued as U.S. Pat. No. 7,775,301 on Aug. 17, 2010, thedisclosures of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present application is generally related to steering tools forhorizontal directional drilling and, more particularly, to a system andmethod using supplemental magnetic information in a steering tool typearrangement.

A boring tool is well-known as a steerable drill head that can carrysensors, transmitters and associated electronics. The boring tool isusually controlled through a drill string that is extendable from adrill rig. The drill string is most often formed of drill pipe sections,which may be referred to hereinafter as drill rods, that are selectivelyattachable with one another for purposes of advancing and retracting thedrill string. Steering is often accomplished using a beveled face on thedrill head. Advancing the drill string while rotating should result inthe drill head traveling straight forward, whereas advancing the drillstring with the bevel oriented at some fixed angle will result indeflecting the drill head in some direction.

One approach that has been taken by the prior art for purposes ofmonitoring the progress of a boring tool in the field of horizontaldirectional drilling, resides in what is commonly referred to as a“steering tool”. This term has come to describe an overall system whichessentially predicts the position of the boring tool, as it is advancedthrough the ground using a drill string, such that the boring tool canbe steered toward a desired target or along a planned drill path withinthe ground. Steering tool systems are considered as being distinct fromother types of locating systems used in horizontal directional drillingat least for the reason that the position of the boring tool ismonitored in a step-wise fashion as it progresses through the ground.For this reason, positional error can accumulate with increasingprogress through the ground up to unacceptable levels.

Generally, in a steering tool system, pitch and yaw angles of thedrill-head are measured in coordination with extension of the drillstring. From this, the drill-head position coordinates are obtained bynumerical integration. Nominal or measured drill rod lengths can serveas a step size during integration. While this method appears to be soundand might enable an experienced driller to use the steering toolsuccessfully, there are a number of concerns with respect to itsoperation, as will be discussed immediately hereinafter.

With respect to the aforementioned positional error, it is noted thatthis error can be attributed, at least in part, to pitch and yawmeasurement errors that accumulate during integration. This can oftenresult in large position errors after only a few hundred feet ofdrilling.

Another concern arises with respect to underground disturbances of theearth's magnetic field, which can cause significant yaw measurement biaserrors, potentially leading to very inaccurate position estimates.

Still another concern arises to the extent that steering effectivenessof a typical HDD drill bit depends on many factors including drill bitdesign, mud flow rate and soil conditions. For example, attempting tosteer in wet and sandy soil with the tool in the 12 o'clock rollposition might become so ineffective that measured pitch does notprovide correct vertical position changes. That is, the orientation ofdrill head, under such drilling conditions, does not necessarily reflectthe direction of its travel.

One approach in dealing with the potential inaccuracy of the steeringtool system is to confirm the position of the drill head independently.For example, the drill head can be fitted with a dipole transmitter. Awalk over locator can then be used to receive the dipole field andindependently locate the drill head. This approach is not alwayspractical, for example, when drilling under a river, lake or freeway. Inthese situations, the operator might notice position errors too lateduring drilling and consequently might not have an opportunity toimplement a drill-path correction.

The foregoing examples of the related art and limitations relatedtherewith are intended to be illustrative and not exclusive. Otherlimitations of the related art will become apparent to those of skill inthe art upon a reading of the specification and a study of the drawings.

SUMMARY

The following embodiments and aspects thereof are described andillustrated in conjunction with systems, tools and methods which aremeant to be exemplary and illustrative, not limiting in scope. Invarious embodiments, one or more of the above-described problems havebeen reduced or eliminated, while other embodiments are directed toother improvements.

In general, a system and associated method are described in which asteering tool is movable by a drill string and steerable in a way thatis intended to form an underground bore along an intended path,beginning from a starting position.

In one aspect, a sensing arrangement, forming one part of the steeringtool, detects a pitch orientation and a yaw orientation of the steeringtool at a series of spaced apart positions of the steering tool alongthe underground bore, each of which spaced apart positions ischaracterized by a measured extension of the drill string. At least onemarker is positioned proximate to the intended path, for transmitting arotating dipole field such that at least a portion of the intended pathis exposed to the rotating dipole field. A receiver, forming anotherpart of the steering tool, receives the rotating dipole field with thesteering tool at a current one of the spaced apart positions to producemagnetic information. A processor is configured for using the detectedpitch orientation, the detected yaw orientation and the measuredextension of the drill string in conjunction with the magneticinformation, corresponding to the current one of the positions of thesteering tool, to determine a current location of the steering tool,relative to the starting position, with a given accuracy such that usingonly the detected pitch orientation, the detected yaw orientation andthe measured extension of the drill string to determine the currentposition, without the magnetic information, would result in a reducedaccuracy in the determination of the current location, as compared tothe given accuracy.

In another aspect, a sensing arrangement is provided, forming one partof the steering tool, for detecting a pitch orientation and a yaworientation of the steering tool. The steering tool is movedsequentially through a series of spaced apart positions along theunderground bore. Each of the spaced apart positions is characterized bya measured extension of the drill string. At least one marker isarranged, proximate to the intended path, for transmitting a rotatingdipole field such that at least a portion of the intended path isexposed to the rotating magnetic dipole. The dipole field is receivedusing a receiver that forms another part of the steering tool, with thesteering tool at a current one of the spaced apart positions on theportion of the intended path, to produce magnetic information. Aprocessor is configured for using the detected pitch orientation, thedetected yaw orientation and the measured extension of the drill stringin conjunction with the magnetic information, corresponding to thecurrent one of the positions of the steering tool, to determine acurrent location of the steering tool relative to the starting positionwith a given accuracy such that using only the detected pitchorientation, the detected yaw orientation and the measured extension ofthe drill string to determine the current location, without the magneticinformation, results in a reduced accuracy in the determination of thecurrent location, as compared to the given accuracy.

In still another aspect, a sensing arrangement is provided, forming onepart of the steering tool, for detecting a pitch orientation and a yaworientation of the steering tool. The steering tool is movedsequentially through a series of spaced apart positions to form theunderground bore. Each of the spaced apart positions is characterized bya measured extension of the drill string, a detected pitch orientationand a detected yaw orientation. At least one portion of the intendedpath is identified along which an enhanced accuracy of a determinationof the current location of the steering tool is desired. One or moremarkers is arranged proximate to the portion of the intended path, eachof which transmits a rotating dipole field such that at least theportion of the intended path is exposed to one or more rotating dipolefields. A receiver is provided, as part of the steering tool, forgenerating magnetic information responsive to the rotating dipolefields. A processor is configured for operating in a first mode usingthe detected pitch orientation, the detected yaw orientation and themeasured extension of the drill string to determine a current locationof the steering tool corresponding to any given one of the spaced apartpositions with at least a given accuracy and for defaulting to a secondmode using the detected pitch orientation, the detected yaw orientation,the measured extension of the drill string and the magnetic information,when the magnetic information is received, to determine the currentlocation of the steering tool with an enhanced accuracy that is greaterthan the given accuracy.

In yet another aspect, a method for establishing a customized accuracyin determination of a position of the steering tool with respect to theintended path is described. A sensing arrangement, forming one part ofthe steering tool, detects a pitch orientation and a yaw orientation ofthe steering tool. The steering tool is moved sequentially through aseries of spaced apart positions to form the underground bore. Each ofthe spaced apart positions is characterized by a measured extension ofthe drill string, a detected pitch orientation and a detected yaworientation. One or more portions of the intended path are identifiedalong which an enhanced accuracy of the determination of the currentlocation of the steering tool is desired. One or more markers arearranged proximate to each one of the portions of the intended pathwhere each of the markers transmits a rotating dipole field such thateach one of the identified portions of the intended path is exposed toone or more rotating dipole fields. As a result of the transmissionrange of the rotating dipole field, more than just those portions of theintended path may be exposed to the rotating dipole field(s). A receiveris provided, as part of the steering tool, for generating magneticinformation responsive to the rotating dipole fields. A processor isconfigured for operating in a first mode using the detected pitchorientation, the detected yaw orientation and the measured extension ofthe drill string to determine a current location of the steering toolcorresponding to any given one of the spaced apart positions with atleast a given accuracy and for operating in a second mode using thedetected pitch orientation, the detected yaw orientation, the measuredextension of the drill string and the magnetic information to determinethe current location of the steering tool with an enhanced accuracy thatis greater than the given accuracy, at least for the one or moreportions of the intended path, to customize an overall positiondetermination accuracy along the intended path.

In addition to the exemplary aspects and embodiments described above,further aspects and embodiments will become apparent by reference to thedrawings and by study of the following descriptions.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments are illustrated in referenced figures of thedrawings. It is intended that the embodiments and figures disclosedherein are to be illustrative rather than limiting.

FIG. 1 is a diagrammatic view, in elevation, of a system according tothe present disclosure operating in a region.

FIG. 2 is a diagrammatic plan view of the system of FIG. 1 in theregion.

FIG. 3a is a block diagram which illustrates one embodiment of asteering tool that is useful in the system of FIGS. 1 and 2.

FIG. 3b is a diagrammatic view, in perspective, of a marker that isuseful in the system of FIGS. 1 and 2.

FIG. 3c shows a coordinate system in which pitch and yaw areillustrated.

FIGS. 4 and 5 illustrate one embodiment of a setup technique that can beused in conjunction with the system of FIGS. 1 and 2.

FIG. 6 is a diagrammatic view, in elevation, of a drill path along whichthe steering tool is disposed, shown here to illustrate one embodimentof a technique for providing an initial solution estimate for theposition of the steering tool.

FIG. 7a is a diagrammatic, further enlarged view, of a portion of FIG.6, shown here to illustrate further details of the initial solutionestimate technique.

FIG. 7b is a flow diagram which illustrates one possible embodiment of atechnique for determining the position of the steering tool using aKalman filter.

FIG. 8 is a plot of random distance error versus distance.

FIGS. 9a and 9b are plots of pitch angle and yaw angle, respectively,versus drill string length for use in a detailed simulation.

FIG. 10a is a plot in a simulation of estimated Y (lateral) steeringtool position with respect to X position, employing a basic steeringtool without the use of markers.

FIG. 10b is a plot in the simulation of estimated Z (elevational)steering tool position with respect to X position, employing a basicsteering tool without the use of markers.

FIG. 10c is a plot, for the simulation of FIGS. 10a and 10b , ofsteering tool coordinate position error versus X position, whichillustrates positional errors for the X, Y and Z axes without the use ofmarkers.

FIG. 11a is a plot in a simulation of estimated Y (lateral) steeringtool position with respect to X position, employing a steering tool inconjunction with one marker.

FIG. 11b is a plot in the simulation of estimated Z (elevational)steering tool position with respect to X position, employing thesteering tool in conjunction with one marker.

FIG. 11c is a plot, for the simulation of FIGS. 11a and 11b , ofsteering tool coordinate position error versus X position, whichillustrates positional errors for the X, Y and Z axes with the use ofone marker.

FIG. 12a is a plot in a simulation of estimated Y (lateral) steeringtool position with respect to X position, employing a steering tool inconjunction with two markers.

FIG. 12b is a plot in the simulation of estimated Z (elevational)steering tool position with respect to X position, employing thesteering tool in conjunction with two markers.

FIG. 12c is a plot, for the simulation of FIGS. 12a and 12b , ofsteering tool coordinate position error versus X position, whichillustrates positional errors for the X, Y and Z axes with the use oftwo markers.

FIG. 13a is a plot in a simulation of estimated Y (lateral) steeringtool position with respect to X position, employing a steering tool inconjunction with three markers.

FIG. 13b is a plot in the simulation of estimated Z (elevational)steering tool position with respect to X position, employing thesteering tool in conjunction with three markers.

FIG. 13c is a plot, for the simulation of FIGS. 13a and 13b , ofsteering tool coordinate position error versus X position, whichillustrates positional errors for the X, Y and Z axes with the use ofthree markers.

FIGS. 14a-c are plots of position error estimates, available through theKalman filter analysis, versus the X axis and directly compared withposition error plots show in FIG. 13c for the drill path of FIGS. 13aand 13 b

FIG. 15 is a diagrammatic plan view of a drilling region for aconcluding portion of an intended drill path, shown here to illustratevarious aspects of arranging and moving markers along the drill path.

FIG. 16 is a diagrammatic plan view of the drilling region and drillpath of FIG. 16, shown her to illustrate further aspects with respect toarranging and moving markers along the drill path.

DETAILED DESCRIPTION

The following description is presented to enable one of ordinary skillin the art to make and use the invention and is provided in the contextof a patent application and its requirements. Various modifications tothe described embodiments will be readily apparent to those skilled inthe art and the generic principles taught herein may be applied to otherembodiments. Thus, the present invention is not intended to be limitedto the embodiment shown, but is to be accorded the widest scopeconsistent with the principles and features described herein, includingmodifications and equivalents, as defined within the scope of theappended claims. It is noted that the drawings are not to scale and arediagrammatic in nature in a way that is thought to best illustratefeatures of interest. Descriptive terminology such as, for example,upper/lower, right/left, front/rear top/bottom, underside and the likehas been adopted for purposes of enhancing the reader's understanding,with respect to the various views provided in the figures, and is in noway intended as being limiting.

Turning now to the figures, wherein like components are designated bylike reference numbers whenever practical, attention is immediatelydirected to FIGS. 1 and 2, which illustrate an advanced steering toolsystem that is generally indicated by the reference number 10 andproduced according to the present disclosure. FIG. 1 is a diagrammaticelevation view of the system, whereas FIG. 2 is a diagrammatic plan viewof the system. System 10 includes a drill rig 18 having a carriage 20received for movement along the length of an opposing pair of rails 22which are, in turn, mounted on a frame 24. A conventional arrangement(not shown) is provided for moving carriage 20 along rails 22. Asteering tool 26 includes an asymmetric face 28 and is attached to adrill string 30 which is composed of a plurality of drill pipe sections32. An intended path 40 of the steering tool includes positions that aredesignated as k and k+1. The steering tool is advanced from position kto k+1 by either a full or a fraction rod length. If very short drillpipe sections are used, the distance between positions k and k+1 couldbe greater than a rod length. By way of example, drill pipe sectionshave a rod length of two feet would be considered as very short. Thesteering tool is shown as having already passed through points 1 and 2,where point 1 is the location at which the steering tool enters theground at 42, serving as the origin of the master coordinate system.While a Cartesian coordinate system is used as the basis for the mastercoordinate systems employed by the various embodiments disclosed herein,it is to be understood that this terminology is used in thespecification and claims for descriptive purposes and that any suitablecoordinate system may be used.

An x axis 44 extends from entry point 42 to a target location T that ison the intended path of the steering tool, as seen in FIG. 1 andillustrated as a rectangle, while a y axis 46 extends to the left whenfacing in the forward direction along the x axis, as seen in FIG. 2. A zaxis 48 extends upward, as seen in FIG. 1. Further descriptions will beprovided at an appropriate point below with respect to establishing thiscoordinate system.

As the drilling operation proceeds, respective drill pipe sections,which may be referred to interchangeably as drill rods, are added to thedrill string at the drill rig. For example, a most recently added drillrod 32 a is shown on the drill rig in FIG. 2. An upper end 50 of drillrod 32 a is held by a locking arrangement (not shown) which forms partof carriage 20 such that movement of the carriage in the directionindicated by an arrow 52 causes section 32 a to move therewith, whichpushes the drill string into the ground thereby advancing the boringoperation. A clamping arrangement 54 is used to facilitate the additionof drill pipe sections to the drill string. The drilling operation iscontrolled by an operator (not shown) at a control console 60 whichitself can include a telemetry section 62 connected with a telemetryantenna 64, a display screen 66, an input device such as a keyboard 68,a processor 70, and a plurality of control levers 72 which, for example,control movement of carriage 20.

Turning now to FIG. 3a , an electromechanical block diagram is shown,illustrating one embodiment of steering tool 26 that is configured inaccordance with the present disclosure. Steering tool 26 includes aslotted non-magnetic drill tool housing 100. A triaxial magnetic fieldsensing arrangement 102 is positioned in housing 100. For this purpose,a triaxial magnetometer or coil arrangement may be used depending onconsiderations such as, for example, space and accuracy. A triaxialaccelerometer 104 is also located in the housing. Outputs from magneticfield sensing arrangement 102 and accelerometer 104 are provided to aprocessing section 106 having a microprocessor at least for use indetermining a pitch orientation and a yaw heading of the steering tool.A dipole antenna and associated transmitter 108 are optionally locatedin the steering tool which can be used, responsive to the processingsection, for telemetry purposes, for transferring encoded data such asroll, pitch, magnetometer readings and accelerometer readings to aboveground locations such as, for example, telemetry receiver 62 (FIG. 1) ofconsole 60 via a dipole electromagnetic field 110 and for locatingdeterminations such as, for example, determining a distance to thesteering tool. For such locating determinations, dipole electromagneticfield 110 can be used in conjunction with a walkover locator, althoughthis is not a requirement and is not practical in some cases, asdiscussed above. Generally, the dipole axis of the dipole antenna isoriented coaxially with an elongation axis of the steering tool in amanner which is well-known in the art. Of course, all of these functionsare readily supported by processing section 106, which reads appropriateinputs from the magnetometer and accelerometer, performs any necessaryprocessing and then performs the actual encoding of information that isto be transmitted.

In another embodiment, processing section 106 is configured forcommunication with processor 70 (FIG. 1) of console 60 using awire-in-pipe approach wherein a conductor is provided in drill string 30for transferring information above ground as described, for example, incommonly owned U.S. Pat. No. 6,223,826 entitledAUTO-EXTENDING/RETRACTING ELECTRICALLY ISOLATED CONDUCTORS IN ASEGMENTED DRILL STRING, which is incorporated by reference in itsentirety. The conductor in the drill string is in electricalcommunication with a line 112 that is in electrical communication withprocessing section 106. It is noted that this approach may also be usedto provide power to a power supply 114 from above ground, as analternative or supplemental to the use of batteries.

Still referring to FIG. 3a , regulated power supply 114, which may bepowered using batteries or through the aforedescribed wire-in-pipearrangement, provides appropriate power to all of the components in thesteering tool, as shown. It is noted that magnetic field sensor 102 canbe used to measure the field generated by a rotating magnet as well asmeasuring the Earth's magnetic field. The later may be thought of as aconstant, much like a DC component of an electrical signal. In thisinstance, the Earth's magnetic field may be used advantageously todetermine a yaw heading.

Referring again to FIGS. 1 and 2, system 10 is illustrated having threemarkers 140 a-c, each of which includes a rotating magnet for generatinga rotating dipole field. Markers 140 a and 140 b are arranged along aline that is generally orthogonal to the X axis, while marker 140 c isoffset toward drill rig 18. A rotating dipole field can be generatedeither by a rotating magnet or by electromagnetic coils. Throughout thisdisclosure, the discussion may be framed in terms of a rotating magnet,but the described applications of magnets carry over to coils and wireloops with only minor modifications. As will be described in furtherdetail, markers can be placed along the drill-path so that they are atleast generally close to the target or other points of interest wherehigh positioning accuracy is required, although one or two markers mayprovide sufficient accuracy for many drilling applications. That is, themarker signal should be receivable by the steering tool along a portionof the intended path including the target or other point(s) of interest.Aside from this consideration, the position of each marker can bearbitrary. Markers can be placed on the ground, on an elevated structureor even lowered within the ground. In each case, the marker can be at anarbitrary angular orientation. The rotation frequency (revolutions persecond) of each magnet can be on the order of 1 Hz, but dipole fieldfrequencies should be distinguishable if more than one marker/magnet isin use. A frequency difference of at least 0.5 Hz is considered to beacceptable for this purpose. Each magnet emits a rotating magneticdipole field whose total flux is recorded by the steering toolmagnetometer and subsequently converted to distance between magnet andtool. During rotation, the magnet of each marker emits a time dependentmagnetic dipole field that is measured by the tri-axial magnetometer ofthe steering tool. As will be seen, a minimum value of the recordedtotal flux provides a distance between each marker and the steeringtool.

Turning now to FIG. 3b , one embodiment of marker 140 isdiagrammatically illustrated. It is noted that aforedescribed markers140 a-c may be of this design as well as any additional markers usedhereinafter. In this embodiment, each marker 140 can include a drivemotor 142 having an output shaft 144 which directly spins a magnet 146having a north pole, which is visible. Motor 142 is electrically drivenby a motor controller 148 to provide stable rotation of the magnet. Themotor can rotate the magnet slowly, for example, at about 1 revolutionper second (1 Hz), as indicated by arrow 150, thereby emitting arotating magnetic dipole field 152 (only partially shown). It should beappreciated that a relatively wide range of rotational speeds may beemployed, for example, from approximately 0.5 Hz to 600 Hz. In oneembodiment, a proportional-integral-derivative (PID) controller can beused to drive motor 142 with user selectable rotational velocity. It isnoted that such PIDs are commercially available. A benefit associatedwith using lower rotational velocity resides in a decreased influence bylocal magnetic objects such as, for example, rebar. If a higherrotational velocity is desired loop antennas can be used to create therotational field. Further, the rotational velocity can be varied so thatthe fields from various markers are distinguishable when simultaneouslyrotating. A suitable power supply can be used, as will be recognized byone having ordinary skill in the art, such as for example a battery andvoltage regulator, which have not been shown. It should be appreciatedthat there is no need for an encoder, since the specific angle of themagnet, corresponding to a particular measurement position, is notinvolved in making the determinations that are described below. Further,orientation sensors and a telemetry section in marker 140 are notneeded. As will be seen, variation in rotation rate of magnet 146 willintroduce associated positional error. Hence, a desire to increasemeasurement accuracy is associated with increasing the rotationalstability of magnet 146.

Still referring to FIG. 3b , while the axis of rotation of magnet 146 isillustrated as being vertical, this is not a requirement. The axis ofrotation can be horizontal or at some arbitrary tilted orientation.Moreover, positioning of the marker for field use does not requireorienting the marker in any particular way. This remarkable degree offlexibility and ease of positioning these markers is one of the benefitsof the system and method taught herein.

Most conventional applications of the steering tool function rely on anominal value for drill rod length when integrating pitch and yaw todetermine position. In accordance with the present disclosure, however,pitch and yaw can be measured more than once along each drill rod suchthat the distance between successive steering tool measurement positionscan be less than the nominal length of one drill rod. This isparticularly the case when the length of the drill rod is exceptionallylong such as, for example, thirty feet. For this purpose, a laserdistance meter, a potentiometer, an ultrasonic arrangement or some otherstandard distance measurement device can be mounted on the drill rig. Anultrasonic arrangement will be described immediately hereinafter.

Referring again to FIGS. 1 and 2, a drill string measuring arrangementincludes a stationary ultrasonic transmitter 202 positioned on drillframe 18 and an ultrasonic receiver 204 with an air temperature sensor206 (FIG. 2) positioned on carriage 20. It should be noted that thepositions of the ultrasonic transmitter and receiver may be interchangedwith no effect on measurement capabilities. Transmitter 202 and receiver204 are each coupled to processor 70 or a separate dedicated processor(not shown). In a manner well known in the art, transmitter 202 emits anultrasonic wave 208 that is picked up at receiver 204 such that thedistance between the receiver and the transmitter may be determined towithin a fraction of an inch by processor 70 using time delay andtemperature measurements. By monitoring movements of carriage 20, inwhich drill string 30 is either pushed into or pulled out of the ground,and clamping arrangement 54, processor 70 can accurately track thelength of drill string 30 throughout a drilling operation. While it isconvenient to perform measurements in the context of the length of thedrill rods, with measurement positions corresponding to the ends of thedrill rods, it should be appreciated that this is not a requirement andthe ultrasonic arrangement can provide the total length of the drillstring at any given moment in time. Further, the length according to thenumber of drill rods multiplied by nominal rod length can be correlatedto the length that is determined by ultrasonic measurement.

Referring to FIG. 1, control console 60, in this embodiment, serves as abase station to communicate with steering tool 26, to monitor its powersupply, to receive and process steering tool data and to send commandsto the steering tool, if so desired. Determined drill-path positions andestimated position errors can be displayed on display screen 66 formonitoring by the system operator. This functionality may also beextended to a remote base station configuration, for example, by usingtelemetry section 64 to transmit information 210 to a remote basestation 212 for display on a screen 214.

Measured Quantities

The steering method requires measurement of the following variables:

Tool pitch and yaw angles ϕ, β

Distances D_(i) between N_(M) magnets and the steering tool (i=1, . . .N_(M))

Magnet positions (X_(M), Y_(M), Z_(M)), (i=1, . . . N_(M))

Initial tool position (X₁, Y₁, Z₁)

Rod length increments Δs_(k)+1 (k=1, 2, 3, . . . )

Referring to FIGS. 1 and 2, pitch and yaw are measured andmagnet-to-tool distances are determined at a series of tool positionsincluding the initial tool position. Point 1, which is additionallydenoted by the reference number 42, designates the position of drillbegin. The steering tool is currently located at a measurement positionk and is intended to proceed to position k+1. These positions cancorrespond to the end points of a drill rod or to intermediate pointsalong the length of each drill rod. As discussed above, intermediatepoints may be needed, for example, when an exceptionally long drill rodis used such as, for example, 30 feet. Higher accuracy will generally beprovided through the use of relatively more measurement positions. Insome cases, the drill rod length may be sufficiently short that thenumber of drill rods may provide a sufficiently accurate value as to thelength of the drill string. The latter situation may also becharacterized by drill rods having a tolerance in their average lengththat is reasonably close to a nominal value. In some embodiments, theremay be no correspondence between the drill rod length and themeasurement positions, for example, where a measurement system, such asis employed by system 10, is capable of measuring and monitoring anoverall length of the drill string. For purposes of simplicity ofdescription, it will be assumed that the drill rod length is used in theremainder of this description to establish the measurement positions. Itis noted that measurements at each measurement position may be performedon-the-fly while pushing and/or rotating the drill string; however,enhanced accuracy can be achieved by stopping movement of the steeringtool at each of the measurement positions during the measurements. A rodlength increment Δs_(k+1) is defined as the arc-length between toolmeasurement positions (X_(k), Y_(k), Z_(k)) and (X_(k+1), Y_(k+1),Z_(k+1)). The setup of this coordinate system is described immediatelyhereinafter.

Set-Up of Steering System

Referring to FIG. 3c , in conjunction with FIGS. 1 and 2, the origin anddirections of the X, Y, Z-coordinate system can be specified in relationto drill begin point 1 and target T. The location where drilling beginsis a convenient choice for the origin and the direction from thisposition to the projection of the target onto a level plane through theorigin defines the X-coordinate axis. The Z-coordinate axis is positiveupward and the Y-coordinate axis completes a right-handed system. Ifdesired, a different right-handed Cartesian coordinate system or anysuitable coordinate system may be used. In the present example, theformulation constrains the X-axis to be level. As noted above, yaworientation is designated as β measured from the X axis in a level X, Yplane, whereas pitch orientation is designated as ϕ measured verticallyfrom the yawed tool position in the X, Y plane as represented by adashed line in the X, Y plane. FIG. 3c defines pitch and yaw as Eulerangles that require a particular sequence of yaw and pitch rotations inorder to rotate the steering tool from a hypothetical position along theX axis into its illustrated position.

Magnet Position Measurements

The use of an Electronic Distance Measurement device (EDM) is currentlythe quickest and most accurate method of defining the X-coordinate axisand measuring magnet position coordinates. However, using an EDM forthis purpose requires the presence of a surveyor at the HDD job site,which may sometimes be difficult to arrange. Accordingly, any suitablemethod may be used.

As an alternative to an EDM, a laser distance measurement device can beused. Devices of this kind are commercially available with a maximumrange of about 650 feet and a distance measurement accuracy of ⅛ of aninch; the Leica Disto™ laser distance meter is an example of such adevice. The device is placed at the position of drill-begin and pointedat the target to obtain the distance between these two positions. Forshort range measurements, the device can be handheld, but for largerdistances it should be fixedly mounted to focus reliably. When an EDM,laser distance measurement device or similar device is used to determinethe magnet positions, the accuracy of the device itself can be used asthe magnet position error in the context of the discussions below.

Referring to FIGS. 4 and 5, one embodiment of a setup technique isillustrated. FIG. 4 illustrates a diagrammatic plan view of steeringtool 26 positioned ahead of drill begin point 1 with target T arrangedalong the X axis and a marker M1 that is offset from the X axis. Thetarget is located at coordinates X_(t), Y_(t), Z_(t). FIG. 5 illustratesa diagrammatic elevational view of steering tool 26 positioned ahead ofdrill begin point 1 on a surface 230 of the ground. In one embodiment, alaser distance meter (LDM) can be used having a tilt sensor so thathorizontal and vertical distances X_(t), Z_(t) to the target can eitherbe calculated or are directly provided by the LDM. The relative position(ΔX, ΔY, ΔZ) between the target and a marker, M1, located near thetarget can also be measured using the LDM, with a measuring tape or inany other suitable manner. Marker position coordinates can be obtainedby adding position increments to target coordinates, as follows:X _(M1) =X _(t) +ΔX  (1)Y _(M1) =ΔY  (2)Z _(M1) =Z _(t) +ΔZ  (3)

The foregoing procedure can be repeated for any number of markers thatare arranged proximate to the target.

In another embodiment, the position of each marker can be measureddirectly, for example using an EDM, with no need to measure the locationof the target, so long as some other position has been provided thatestablishes the X axis from point 1 of drill begin. For example, amarker M2 may be arranged along the X axis. As will be furtherdescribed, location accuracy along the X axis can be customized based onthe arrangement of markers therealong. The need for enhanced accuracyfor some portion of the path of the steering tool can be established,for example, based on the presence of a known inground obstacle 232.

Reference Yaw Angle

Continuing to refer to FIGS. 4 and 5, a reference yaw angle β_(ref) isdefined as the yaw angle of the steering tool, measured by the steeringtool, with its elongation axis aligned with the X-direction. In thepresent example, the reference yaw angle is measured as a compassorientation from magnetic north, based on the Earth's magnetic field.Since steering tool yaw has previously been defined as positive for acounterclockwise rotation the particular reference yaw angle β_(ref)shown in FIG. 4 is negative. Accordingly, in order to measure yawaccurately without interference from the magnetic influence of the drillrig, the steering tool can be placed on a level ground a sufficientdistance ahead of the drill; 30 feet is usually adequate. The elongationaxis of the steering tool is at least approximately on or at leastparallel to the X-axis. Yaw angle β_(m), measured as a compass headingduring steering, is subsequently replaced by β=β_(m)−β_(ref).

Steering Procedure Formulation

Nomenclature

-   -   c_(A)=pitch and yaw error covariance matrix    -   c_(e)=empirical coefficient    -   c_(M)=magnet position error covariance matrix    -   D=distance between marker and steering tool    -   F=continuous state equations matrix    -   H=observation coefficient vector    -   N_(M)=number of markers    -   P=error covariance matrix    -   Q=continuous process noise covariance parameter matrix    -   Q_(k)=discrete process noise covariance matrix    -   R=observation covariance scalar    -   {right arrow over (r)}=vector of magnet position measurement        error    -   s=arc-length along drill-rod axis    -   v_(D)=distance measurement noise    -   v_(M)=magnet position measurement noise    -   {right arrow over (x)}=state variables vector    -   X, Y, Z=global coordinates    -   X_(k), Y_(k), Z_(k)=steering tool position coordinates    -   z=measurement scalar    -   β=yaw angle    -   δX, δY, δZ=position state variables    -   δX_(M), δY_(M), δZ_(M)=magnet position increments    -   δβ, δϕ=yaw and pitch angle increments    -   Δs=rod length increment    -   ϕ=pitch angle    -   Φ_(k)=discrete state equation transition matrix    -   σ=standard deviation    -   σ²=variance, square of standard deviation

Subscripts

-   -   bias=bias error    -   D=distance    -   i exact value    -   i=i-th magnet    -   k=k-th position on drill path    -   M=magnet    -   m=measured    -   ref=reference    -   1=initial tool position (drill begin at k=1)

Superscripts

$( {}^{\cdot} ) = \frac{d}{ds}$

-   -   ( )⁻=indicates last available estimate    -   ( )′=transpose    -   ( )*=nominal drill path    -   {right arrow over ({circumflex over (x)})}=state variables        vector estimate        Tracking Equations

The method is based on two types of equations, referred to as steeringtool process equations and distance measurement equations. The formerare a set of ordinary differential equations describing how toolposition (X, Y, Z) changes along the drill-path as a function ofmeasured pitch ϕ and yaw β and shown as equations 4.

$\begin{matrix}{\begin{Bmatrix}\overset{.}{X} \\\overset{.}{Y} \\\overset{.}{Z}\end{Bmatrix} = \begin{Bmatrix}{\cos\mspace{14mu}\phi\mspace{14mu}\cos\mspace{14mu}\beta} \\{\cos\mspace{14mu}\phi\mspace{14mu}\sin\mspace{14mu}\beta} \\{\sin\mspace{14mu}\phi}\end{Bmatrix}} & (4)\end{matrix}$

The over-dot indicates that derivatives of position coordinates are tobe taken with respect to arc-length s along the axis of the drill rod.Pitch and yaw angles are illustrated in FIG. 3c . Accordingly, thepremise of a conventional steering tool resides in a numericalintegration of equations 4 with respect to arc length s of the drillstring. Unfortunately, as discussed above, this technique readilyproduces potentially serious positional errors in and by itself.

The aforementioned distance measurement equations are of the form:D ²=(X _(M) −X)²+(Y _(M) −Y)²+(Z _(M) −Z)²  (5)

The distance measurement equations express distance D between the centerof a rotating magnet of a marker and the center of tri-axial steeringtool magnetometer 102 (see FIG. 1) in terms of tool position (X, Y, Z)and magnet position (X_(M), Y_(M), Z_(M)). Accordingly, NV of suchequations can be written for a system, corresponding to the total numberof markers.

The origin of the global X, Y, Z-coordinate system in which toolposition will be tracked can be chosen to coincide with the location ofdrill begin (point 1 in FIGS. 1 and 2).X ₁=0 Y ₁=0 Z ₁=0  (6)

Equations (4), (5) and (6) represent an initial value problem that canbe solved for steering tool position coordinates.

Nonlinear Solution Procedures

The foregoing initial value problem can be solved using either anonlinear solution procedure, such as the method of nonlinear leastsquares, the SIMPLEX method, or can be based on Kalman filtering. Thelatter will be discussed in detail beginning at an appropriate pointbelow. Initially, however, an application of the SIMPLEX method will bedescribed where the description is limited to the derivation of thenonlinear algebraic equations that are to be solved at each drill-pathposition. Details of the solver itself are well-known and considered aswithin the skill of one having ordinary skill in the art in view of thisoverall disclosure.

The present technique and other solution methods can replace thederivatives {dot over (X)}, {dot over (Y)}, Ż in equations (4) withfinite differences that are here written as:

$\begin{matrix}{\overset{.}{X} = \frac{X_{k + 1} - X_{k}}{\Delta\; s_{k + 1}}} & (7) \\{\overset{.}{Y} = \frac{Y_{k + 1} - Y_{k}}{\Delta\; s_{k + 1}}} & (8) \\{\overset{.}{Z} = \frac{Z_{k + 1} - Z_{k}}{\Delta\; s_{k + 1}}} & (9)\end{matrix}$

Resulting algebraic equations read:f ₁ =X _(k+1) −X _(k) −Δs _(k+1) cos ϕ_(k) cos β_(k)=0  (10)f ₂ =Y _(k+1) −Y _(k) −Δs _(k+1) cos ϕ_(k) sin β_(k)=0  (11)f ₃ =Z _(k+1) −Z _(k) −Δs _(k+1) sin ϕ_(k)=0  (12)

The distance measurement equations (5) provide additional N_(M)equations written as:f ₄ _(i) =D _(k+1,i) ²−(X _(k+1) −X _(M) _(i) )²×(Y _(k+1) −Y _(M) _(i))²−(Z _(k+1) −Z _(M) _(i) )²=0  (13)

Starting with the known initial values (Equations 6) at drill begin, thecoordinates of subsequent positions along the drill path can be obtainedby solving the above set of nonlinear algebraic equations (10-13) foreach new tool position. The coordinates of position k+1 are calculatediteratively beginning with some assumed initial solution estimate thatis sufficiently close to the actual location to assure convergence tothe correct position. One suitable estimate will be describedimmediately hereinafter.

Referring to FIGS. 6 and 7 a, the X, Z plane is illustrated with a drillpath 240 formed therein and in a direction 242 using a plurality ofdrill rods 32, at least some of which have been designated by referencenumbers. FIG. 7a is an enlarged view within a dashed circle 244 of FIG.6. An initial solution estimate is given by a point on what may bereferred to as a nominal drill-path 246 that can be found by linearextrapolation of the previously predicted/last determined position to apredicted position 248. The linear extrapolation is based on equations 4and a given incremental movement Δs_(k+1) of the steering tool from ak^(th) position where:

$\begin{matrix}{\begin{Bmatrix}X_{k + 1}^{*} \\Y_{k + 1}^{*} \\Z_{k + 1}^{*}\end{Bmatrix} = {\begin{Bmatrix}X_{k} \\Y_{k} \\Z_{k}\end{Bmatrix} + {\Delta\; s_{k + 1}\begin{Bmatrix}{\cos\mspace{14mu}\phi_{k}\mspace{14mu}\cos\mspace{14mu}\beta_{k}} \\{\cos\mspace{14mu}{\phi\;}_{k}\mspace{11mu}\sin\mspace{14mu}\beta_{k}} \\{\sin\mspace{14mu}\phi_{k}}\end{Bmatrix}}}} & (14)\end{matrix}$

Where predicted positions are indicated in equations 14 using anasterisk ( )*. It should be appreciated that the position of thesteering tool is characterized as predicted or estimated since thelocation is not identified in an affirmative manner such as is the case,for example, when a walk-over locator is used. The use of a steeringtool differs at least for the reason that the position of the steeringtool is estimated or predicted based on its previous positions. Thus,the actual position of the steering tool, for a sufficiently long drillpath, can be significantly different than the position that isdetermined by a steering tool technique, as a result of accumulatingerror, if this error is not managed appropriately.

Application of the SIMPLEX method requires definition of a function thatis to be minimized during the solution procedure. An example of such afunction that is suitable in the present application reads:

$\begin{matrix}{F = {\sum\limits_{p = 1}^{3 + N_{M}}\; f_{p}^{2}}} & (15)\end{matrix}$

As noted above, it is considered that one having ordinary skill canconclude the solution procedure under SIMPLEX in view of the foregoing.

Kalman Filter Solution

In another embodiment, a method is described for solving the trackingequations employing Kalman filtering. The filter minimizes the positionerror caused by measurement uncertainties in a least square sense. Thefilter determines position coordinates as well as position errorestimates.

The three tool position coordinates (X, Y, Z) are chosen as the mainsystem parameters. Increments (δX, δY, δZ) of these parameters arereferred to as state variables. The solution method can be characterizedas a predictor-corrector technique. Assuming all drill-path variablesare known at a last determined position and a drill string increment isknown, the current or next-determined position on the drill path can beapproximated by linear extrapolation, as described above with respect toFIGS. 6 and 7 a. This is the predictor step that gives a point onnominal drill path 246. The Kalman filter, in turn, performs a correctorstep in which state variables are calculated and added to the nominaldrill path.

Initial tool position coordinates (X₁, Y₁, Z₁) are assumed andcorresponding error variances (σ_(X) ₁ ², σ_(Y) ₁ ², σ_(Z) ₁ ²) areknown. For example, at (X₁, Y₁, Z₁), which is the origin of thecoordinate system, the error variances are zero. If (X₁, Y₁, Z₁) is notthe origin, the error variances are based on the accuracy of measurementfrom the origin. The tracking procedure starts from this initialposition and proceeds along the drill path, as follows:

As is illustrated in FIGS. 6 and 7 a, the last known drill path position(X_(k), Y_(k), Z_(k)) is extrapolated linearly to obtain an approximateor estimated tool position, previously introduced as nominal drill pathposition (X_(k+1)*, Y_(k+1)*, Z_(k+1)*).

The filter determines state variables (δX_(k+1), δY_(k+1), δZ_(k+1)) andstandard deviations of position error (σ_(X) _(k+1) , σ_(Y) _(k+1) ,σ_(Z) _(k+1) ).

State variables are added to the nominal drill path position to find thenew tool position (X_(k+1), Y_(k+1), Z_(k+1)).

$\begin{matrix}{\begin{Bmatrix}X_{k + 1} \\Y_{k + 1} \\Z_{k + 1}\end{Bmatrix} = {\begin{Bmatrix}X_{k + 1}^{*} \\Y_{k + 1}^{*} \\Z_{k + 1}^{*}\end{Bmatrix} + \begin{Bmatrix}{\delta\; X_{k + 1}} \\{\delta\; Y_{k + 1}} \\{\delta\; Z_{k + 1}}\end{Bmatrix}}} & (16)\end{matrix}$Measurement Errors

The Kalman filter takes the following random measurement errors intoaccount which must therefore be known before tracking begins.

Tool pitch and yaw angle errors σ_(ϕ), σ_(β)

Distance error σ_(D)

Magnet position errors (σ_(X) _(M) , σ_(Y) _(M) , σ_(Z) _(M) )

Initial tool position errors (σ_(X) ₁ , σ_(Y) ₁ , σ_(Z) ₁ )

Error values are empirical and depend on the type of instrumentationused. Note that the effect of drill rod length measuring error is notpart of the analysis since arc-length along the axis of the drill rod isused as an independent variable.

Knowing initial tool position errors (σ_(X) ₁ , σ_(Y) ₁ , σ_(Z) ₁ ), thecorresponding error covariance matrix P₁ is given as:

$\begin{matrix}{P_{1} = \begin{bmatrix}\sigma_{X_{1}}^{2} & 0 & 0 \\0 & \sigma_{Y_{1}}^{2} & 0 \\0 & 0 & \sigma_{Z_{1}}^{2}\end{bmatrix}} & (17)\end{matrix}$

Adding the latter to equations (4) to (6) completes the formulation ofthe initial value problem to be solved by Kalman filtering.

Linearized Tracking Equations

In addition to various measured quantities that are summarized above,the Kalman filter solution uses input of the following parameters.

Φ_(k) discrete state equation transition matrix

Q_(k) discrete process noise covariance matrix

z measurement scalar

H observation coefficient vector

R observation error covariance scalar

The above parameters are derived by linearizing the steering toolprocess equations and distance measurement equations about the nominaldrill path position. The resulting two sets of linear equations are theso-called state equations and the observation equations. They aresummarized below.

The state variables are defined as position increments.{right arrow over (x)}=(δX,δY,δZ)′  (18a){dot over ({right arrow over (x)})}=(δ{dot over (X)},δ{dot over(Y)},δŻ)′  (18b)

The state equations governing state variables read{right arrow over (x)} _(k+1)=Φ_(k) {right arrow over (x)} _(k) +Δs_(k+1) G _(k) {right arrow over (u)} _(k)  (19)

Where Δs_(k+1)G_(k){right arrow over (u)}_(k) represents pitch and yawmeasurement noise. It is noted that, hereinafter, subscripts may bedropped for purposes of clarity. Accordingly:

$\begin{matrix}{\Phi = I} & (20) \\{{Q = {{cov}( {( {\Delta\; s} )G\overset{arrow}{u}} )}}{and}} & (21) \\{\overset{arrow}{u} = ( {{\delta\phi},{\delta\beta}} )^{\prime}} & (22) \\{G = \begin{bmatrix}{{- \sin}\mspace{14mu}\phi\mspace{11mu}\cos\mspace{14mu}\beta} & {{- \cos}\mspace{14mu}\phi\mspace{14mu}\sin\mspace{14mu}\beta} \\{{- \sin}\mspace{14mu}\phi\mspace{14mu}\sin\mspace{14mu}\beta} & {\cos\mspace{14mu}\phi\mspace{14mu}\cos\mspace{14mu}\beta} \\{\cos\mspace{14mu}\phi} & 0\end{bmatrix}} & (23)\end{matrix}$

The discrete noise covariance matrix Q_(k) becomes:

$\begin{matrix}{c_{A} = \begin{bmatrix}\sigma_{\phi}^{2} & 0 \\0 & \sigma_{\beta}^{2}\end{bmatrix}} & (24) \\{Q = {{c_{e}( {\Delta\; s} )}^{2}{Gc}_{A}G^{\prime}}} & (25)\end{matrix}$

Note that the empirical coefficient c_(e) has been added to equation(25) in order to account for pitch and yaw bias errors. It has unitvalue if pitch and yaw measurement errors are entirely random.

The observation equation of a rotating magnet reads:z=H{right arrow over (x)}+v _(D) +v _(M)  (27)R=cov(v _(D) +v _(M))  (28)

Where the term v_(D) represents distance measurement noise and the termv_(M) represents magnet position measurement noise. The term H will bedescribed at an appropriate point below. The symbol z, seen in equation(27) is a difference between measured distance D and calculated distanceD* from a marker to the nominal drill path position, given as:z=D−D*  (29)D ^(*) ² =(X*−X _(M))²+(Y*−Y _(M))²+(Z*−Z _(M))²  (30)

The first term H on the right hand side of equation (27) is theobservation coefficient vector, written as:

$\begin{matrix}{H = ( {\frac{X^{*} - X_{M}}{D^{*}},\frac{Y^{*} - Y_{M}}{D^{*}},\frac{Z^{*} - Z_{M}}{D^{*}}} )} & (31)\end{matrix}$

The following form of the observation covariance scalar R is used in thesteering tool method:

$\begin{matrix}{R = {\sigma_{D}^{2} + {{Hc}_{M}H^{\prime}}}} & (32) \\{c_{M} = \begin{bmatrix}\sigma_{X_{M}}^{2} & 0 & 0 \\0 & \sigma_{Y_{M}}^{2} & 0 \\0 & 0 & \sigma_{Z_{M}}^{2}\end{bmatrix}} & (33)\end{matrix}$Projection of State Variables and Estimation Errors

An estimate of the state vector at the next steering tool position k+1is denoted by {right arrow over ({circumflex over (x)})} and its errorcovariance matrix is P⁻ where the superscript ( )⁻ indicates the lastavailable estimate. Before the filter is applied at the new toolposition, set{right arrow over ({circumflex over (x)})}={0}  (34)

The error covariance matrix P_(k) is projected to the new position usingP _(k+1) ⁻=(Φ_(k) P _(k)Φ_(k) ′+Q _(k)  (35)Kalman Filter Loop

The filter loop is executed once for each marker, resulting in aflexible arrangement that is able to process any number of markers inuse by the steering tool system.

The classical, well documented version of the filter loop is chosen as abasis for the current steering tool embodiment. It consists of threesteps:

Kalman Gain:K=P ⁻ H′(HP ⁻ H′+R)⁻¹  (36)

State Variables:{right arrow over ({circumflex over (x)})}={right arrow over({circumflex over (x)})}⁻ +K(z−H{right arrow over ({circumflex over(x)})} ⁻)  (37)

Error Covariance Matrix:P=(I−KH)P ⁻  (38)Position Coordinate Errors

Having completed the filter analysis at a new position, its coordinatesare given by equation (16). Corresponding one-sigma position errorsfollow from:σ_(X)=√{square root over (P ₁₁)}  (39)σ_(Y)=√{square root over (P ₂₂)}  (40)σ_(Z)=√{square root over (P ₃₃)}  (41)

FIG. 7b is a flow diagram, generally indicated by the reference number260, which illustrates one embodiment of a Kalman filter implementationaccording to the descriptions above. At 262, the nominal position of thesteering tool at k+1 is determined using equation 14. At 264, the errorcovariance matrix is projected to position k+1 using equation 35. Thestate vector is initialized at 266. Beginning with step 270, a loop isentered using magnetic measurements associated with one marker. Thedistance D* between a point on the nominal drill path and the marker isdetermined per equation 30. The observation coefficient vector H in turnis calculated using equation 31. Equation 32 provides the observationcovariance scalar R. At 272, the Kalman filter is executed usingequations 36-38. At 274, a determination is made as to whether magneticinformation is available that is associated with another marker. If so,execution returns to step 270. If magnetic information from all markershas been processed, step 276 establishes the final coordinates of thecurrent position of the steering tool based on equation 16 and canassociate a position error estimate with these coordinates, based onequations 39-41.

Numerical Simulations

Several numerical simulations were performed to estimate positions ofthe steering tool assisted by up to three rotating magnets. In all casesthe steering tool was tracked, moving along a drill-path defined by:

$\begin{matrix}{0 \leq X_{ex} \leq {300\mspace{14mu}{ft}}} & (42) \\{Y_{ex} = {15{\sin( {\frac{\pi}{300}X_{ex}} )}}} & (43) \\{Z_{ex} = {{- 2}Y_{ex}}} & (44)\end{matrix}$

Note that drilling starts at the origin of the global coordinate system.The steering tool reaches a maximum depth of 30 feet and yaws to theside with a maximum lateral displacement of 15 feet before it reachesthe target 300 feet out. The above coordinates are exactly knowncoordinates from which values for pitch, yaw and tool to magnetdistances were derived.

Table 1 summarizes random and bias errors that were added to these exactvalues to generate “measured” data.

TABLE 1 Errors for Generating “Measured” Simulation Data Pitch Errorσ_(ϕ) = 0.25 deg ϕ_(bias) = 0.25 deg Yaw Error σ_(β) = 0.50 deg β_(bias)= 0.50 deg Drill Rod Length Error σ_(Δs) = 0.01 ft Distance ErrorD_(bias) = 0.02 ft (See also, FIG. 8)

FIG. 8 sets forth random distance error σ_(D) in feet, plotted againstdistance D in feet. It is noted that errors were chosen based onempirical measurements with specific pitch and yaw sensors as well aswith rotating magnets. Table 2 summarizes the random errors used asinput for the filter. Note that the rod length increment error is usedonly for generating measured data; it is not used by the filter.

TABLE 2 Random Errors Used in Kalman Filter Pitch Error σ_(ϕ) = 0.5 degYaw Error σ_(β) = 1 deg Distance Error σ_(D) (see FIG. 8) MagnetPosition Errors σ_(X) _(M) = σ_(Y) _(M) = σ_(Z) _(M) = 0.02 ft InitialPosition Error σ_(X) ₁ = σ_(Y) ₁ = σ_(Z) ₁ = 0

FIGS. 9a and 9b are plots against drill string length, in feet, whichcompare exact with “measured” pitch and yaw angles, respectively, usedin all the simulations described below. Exact pitch and yaw values areshown by dotted lines, while measured pitch and yaw values are shown bysolid lines. Increments between adjacent measurement positions along thedrill-path were approximately three feet.

Estimated steering tool positions and position errors are illustrated byFIGS. 10a-c , as an application of the basic steering tool functionwithout the use of markers. It is noted that, in subsequent figures, anincreasing number of magnets is added to the system to demonstrate theimprovements that are provided through the use of markers. Illustratedposition errors are shown as the differences between estimated and exactvalues. Since “measured” values for pitch and yaw contain bias as wellas random components, lateral and vertical position errors are alsobiased. FIG. 10a is a diagrammatic plan view of the estimated drillpath, designated by the reference number 300, whereas FIG. 10b is anelevational view of the estimated drill path, designated by thereference number 302. FIG. 10c illustrates the X coordinate positionalerror as a solid line 310, the Y coordinate positional error as a dashedline 312 and the Z coordinate positional error as a dotted line 314. Inthe present example, without the use of magnets, it can be seen thatthere is a continuously accumulating Y coordinate error, which increasesto about three feet upon reaching X=300 feet, the X axis coordinate oftarget T. The Z coordinate error is over one foot.

Referring collectively to FIGS. 11a-c , simulations are now presentedincluding the use of markers. One marker 320 is used at a location ofX=300 ft, Y=−5 ft, and Z=5 ft. FIG. 11a is a diagrammatic plan view ofthe estimated drill path, designated by the reference number 322,whereas FIG. 11b is an elevational view of the estimated drill path,designated by the reference number 324. FIG. 11c illustrates the Xcoordinate positional error as a solid line 326, the Y coordinatepositional error as a dashed line 328, and the Z coordinate positionalerror as a dotted line 330. In the present example, with the use of onlyone magnet near target T, it can be seen that the Y coordinate error isdramatically reduced to just over one foot upon reaching the target Xcoordinate at 300 feet.

Referring collectively to FIGS. 12a-c , a second marker 340 is added ata location of X=305 ft, Y=0 ft and Z=5 ft. FIG. 12a is a diagrammaticplan view of the estimated drill path, designated by the referencenumber 342, whereas FIG. 12b is an elevational view of the estimateddrill path, designated by the reference number 344. FIG. 12c illustratesthe X coordinate positional error as a solid line 346, the Y coordinatepositional error as a dashed line 348, and the Z coordinate positionalerror as a dotted line 350. In the present example, with the use of twomagnets near target T, it can be seen that the Y coordinate error isstill further reduced to a relatively small fraction of one foot uponreaching the target X coordinate at 300 feet. Moreover, the X and Zcoordinate errors are likewise reduced to a small fraction of one footupon reaching the target X coordinate at 300 feet.

Referring collectively to FIGS. 13a-c , a third marker 360 is added at alocation of X=300 ft, Y=5 ft and Z=5 ft. FIG. 13a is a diagrammatic planview of the estimated drill path, designated by the reference number362, whereas FIG. 13b is an elevational view of the estimated drillpath, designated by the reference number 364. FIG. 12c illustrates the Xcoordinate positional error as a solid line 366, the Y coordinatepositional error as a dashed line 368, and the Z coordinate positionalerror as a dotted line 370. In the present example, with the use ofthree magnets near target T, it can be seen that the X, Y and Zcoordinate errors are reduced to a very small fraction of one foot uponreaching the target X coordinate at 300 feet. In view of the foregoing,the use of two or three markers proximate to a point of interest on thedrill path (such as the target) enables a high precision guidance of thesteering tool to a target at least 300 feet out from the point of drillbegin, or enables high precision steering relative to some point ofinterest along the drill path at least 300 feet out.

It should be appreciated that, in the aforedescribed numericalsimulations, errors defined as the difference between estimated andexact positions can be calculated, since exact drill-path coordinatesare known. This type of error can not be calculated during actualdrill-head tracking. Accordingly, a different type of error estimate isused for actual drilling. The Kalman filter analysis provides such anerror estimate in the form of standard deviations of positioncoordinates. In this regard, FIGS. 14a-c illustrate the two types ofposition errors for the drill-path of FIGS. 13a-b with three markersplaced near the target. The solid lines denote the +1 sigma positionerror provided by the Kalman filter analysis, whereas the dashed linesrepresent the corresponding −1 sigma errors. For comparison, theposition errors of FIG. 13c , defined as the difference betweenestimated and exact positions, are also shown in FIGS. 14a-c . As seen,position errors expressed in terms of standard deviations vary smoothlyalong the drill-path since they are based on a statistical measure. Incontrast, estimated positions and, hence, the errors shown as dottedlines in FIGS. 14a-c are based on one set of partly random measurementsresulting in an irregular distribution of position errors. Repeating theKalman filter analysis with a different set of random measurements wouldproduce different error distributions of this type. Numericalsimulations were performed with c_(e)=16.

Attention is now directed to FIGS. 15 and 16 for purposes of describingadditional aspects of the present disclosure. FIG. 15 illustrates a planview of a drilling region 400 having a concluding section of an intendeddrill path 402 defined therein. Further, a first inground obstacle 404and a second inground obstacle 406 are shown in relation to intendedpath 402. As can be seen, intended path 402 has been specificallydesigned to avoid inground obstacles 404 and 406. Such path design canbe based on any knowledge of inground features that should be avoidedand can include a reliance on any suitable resource including but notlimited to utility surveys, available design drawings and exploratoryexcavations. Moreover, inground obstacles 404 and 406 are intended torepresent any type of feature within the ground that should be avoided.

Still referring to FIGS. 14 and 15, an exemplary plurality of markers140 a-e is distributed along intended path 402 such that markers 140 aand 140 b are in the vicinity of obstacle 404, marker 104 c is in thevicinity of obstacle 406, and markers 140 d and 140 e are in thevicinity of target T. It should be appreciated that orientation of themarkers is arbitrary so long as the steering tool, on the intended pathand proximate to some inground feature of interest, is capable ofreceiving at least the magnetic field that is emanated by the markers inits general vicinity. As seen above, with each marker that is addedproximate to target T, there is a corresponding increase in steeringtool accuracy. That is, the steering tool tracks the intended path withproportionally increasing accuracy. Placement of markers proximate topoints of interest, as illustrated, likewise produces a correspondingincrease in accuracy along any portion of the intended path that isexposed to the magnetic field that is emanated by that marker. In thisway, an enhanced steering accuracy, of a selective degree, can beprovided at any desired point or points along the intended path.Accordingly, a highly advantageous customized steering accuracy isprovided along the intended path. In this regard, as discussed above,the described technique readily accommodates receiving signals from anynumber of markers at any given point along the intended path orreceiving no marker signals for some portions of the path, such as mightbe the case at a point 410 midway between markers 140 c and 140 d of thepresent example.

Even though the present example illustrates the use of five markers,fewer markers may actually be necessary since the markers can be movedalong the intended drill path responsive to the progression of thesteering tool. For example, after the steering tool passes obstacle 404,marker 140 a can be moved to the position of marker 140 c. At a suitabletime, marker 140 b can be moved to the position of marker 140 d. Oncethe steering tool passes obstacle 406, marker 140 a can then be moved tothe illustrated position of marker 140 e. Accordingly, long drill runscan be made with as few as one or two markers.

Applicants consider that sweeping advantages are provided over thestate-of-the-art with respect to steering tool systems and methods.While there are systems in the prior art that use rotating magnetsignals, it should be apparent from the detailed descriptions above thatproviding the capability to use rotating magnet signals in the contextof a steering tool system is neither trivial nor obvious. In thisregard, Applicants are unaware of any prior art use of a rotating magnetsignal in the context of a steering tool system and, particularly, withsuch flexibility and ease of use where the rotating magnet field markerscan not only be arbitrarily placed, but arbitrarily oriented.

While a number of exemplary aspects and embodiments have been discussedabove, those of skill in the art will recognize certain modifications,permutations, additions and sub-combinations thereof. It is thereforeintended that the following appended claims and claims hereafterintroduced are interpreted to include all such modifications,permutations, additions and sub-combinations as are within their truespirit and scope.

What is claimed is:
 1. In a system including a steering tool that ismoved by a drill string and steered in a way that is intended to form anunderground bore along an intended path, beginning from a startingposition, an apparatus comprising: a sensing arrangement, forming onepart of the steering tool, for detecting a pitch orientation and a yaworientation of the steering tool at a series of spaced apart positionsof the steering tool along said underground bore, each of which spacedapart positions is characterized by a measured extension of said drillstring such that each of the detected pitch orientation and yaworientation are subject to a measurement error; at least one markerpositioned proximate to the intended path, for transmitting a rotatingdipole field such that at least a portion of the intended path isexposed to the rotating dipole field; a receiver, forming another partof the steering tool, for receiving the rotating dipole field with thesteering tool at a current one of said spaced apart positions to producemagnetic information; and a processor that is configured for using thedetected pitch orientation, the detected yaw orientation and themeasured extension of the drill string, in conjunction with the magneticinformation, to determine a current location of the steering toolrelative to a last determined position while compensating for themeasurement error, with an accuracy that is greater than if the magneticinformation was not used with the detected pitch orientation, thedetected yaw orientation and the measured extension of the drill stringand said measurement error was allowed to accumulate with eachsuccessive position.
 2. The apparatus of claim 1 wherein said processoris further configured for operating in a first mode using the detectedpitch orientation, the detected yaw orientation and the measuredextension of the drill string to determine the current location of thesteering tool corresponding to any given one of the spaced apartpositions with a reduced accuracy that is subject to said measurementerror as said measurement error accumulates with each successiveposition in said series of positions and for defaulting to a second modeusing the detected pitch orientation, the detected yaw orientation, themeasured extension of the drill string and the magnetic information,when the magnetic information is received along said portion of theintended path, to determine the current location of the steering toolwith said accuracy that is greater than said reduced accuracy tocompensate for the accumulation of said measurement error.
 3. Theapparatus of claim 1 wherein said processor is further configured to usea set of ordinary differential equations to determine a change in theposition of the steering tool along the drill-path as a function ofmeasured pitch and yaw and as part of determining the current locationof the steering tool.
 4. The apparatus of claim 3 wherein said processoris further configured to use a distance measurement equation inconjunction with said set of ordinary differential equations with thedistance equation representing a distance between said marker and saidreceiver of the steering tool as part of determining the currentlocation of the steering tool.